Her words may never have been found if not for the relentless determination of her family, who never gave up on her. Carly's story is how one child found her way out of the dense forest that is autism, and how her experience may unlock the mysteries of this baffling disorder.

Diagnosed with Autism, Carly Lost in Own World

Born in Toronto, Carly was 2 years old when it became clear she wasn't keeping up with her twin sister, Taryn. When her parents, Arthur and Tammy Fleischmann, learned the diagnosis was autism they expected the worst -- that one of their twins would never achieve milestones most take for granted.

"When you're told your child is going to be developmentally delayed, that they might achieve the developmental level of a 6-year-old, it's like being kicked in the gut," Carly's father recalls. "And so for us, we have expectations of one child who's going to grow up and be independent and accomplish things that she wants in life, and one child, a big mystery, what's going to become of her."

In the beginning, Carly's delays prevented her from walking and sitting up, but as she grew, it became painfully clear that Carly couldn't speak. Like most autistic children, she was lost in her own world, perpetually swimming under water.

Experts told the Fleischmanns that early intervention was critical, and since Carly was 3, her therapy has been intensive and unrelenting, sometimes working with three to four different therapists daily for up to 60 hours each week. Her team, nicknamed Carly Inc., would constantly change therapies that weren't working and streamline the ones that did. Carly made small gains, but nothing that ever seemed like real progress.

"When you look in Carly's eyes, you can see an innate intelligence. So we never gave up," her father recalls.

But if there was intelligence, Carly's ceaseless rocking, flailing arms and tantrums hid any trace of it. Worse, she couldn't speak, not one word.

Intensive Therapy Attempts to Find Way for Carly to Communicate

For many years, Nicole Walton-Allen, a clinical psychologist, led Carly's therapy program.

"Her profile was that of a child who was severely autistic and more than likely moderate mentally retarded," Walton-Allen told "20/20." "I did not have any expectation that she would have a fluent form of communication. She was getting up and running around. Her hands were constantly in motion, flapping. She was drooling."